Human tremor is an involuntary trembling or shaking of the muscles of the body associated with physical weakness, emotional stress, or excitement. Tremor can detrimentally affect task performance for a number of different tasks including microsurgery, marksmanship, photography, videotaping and microcircuit assembly as well as the simple use of a pen by a person afflicted with mild Parkinson's disease.
As an example, surgeons are often required to perform surgery on patients in areas which are very delicate and leave little or no room for error. One such example is eye surgery. A surgeon performing surgery on the eye by cutting the eye with a laser or a scalpel must be very accurate, as a cut which is too deep, or in the wrong position, due to tremor of the surgeon's hand, can be disastrous.
Another example is a marksman such as a military or law enforcement sniper. Such individuals must often make tremendously accurate shots in a split second in order to save the lives of others. However, human tremor resulting from the stress and excitement of the situation as well as the physical fatigue resulting from supporting the weapon for extended periods of time while waiting for the opportunity to make a shot can take its toll on a shooter so that when the time arrives to make the shot, the shot misses its mark. A missed opportunity, whether missing the target completely or merely wounding the target, can further endanger the lives of others.
One technique for compensating for human tremor in such a situation is to fire the weapon, via a computer, as the target moves through the cross hairs. However, this takes the "trigger" away from the shooter, and thus the weapon may fire too early or too late. Moreover, this technique does not solve the problem, but only masks it. The weapon still moves due to the tremor and no stabilization is provided.
One open loop system intended to compensate for jitter due to human tremor is provided for binoculars. This open loop system includes a deformable prism or a steering prism and mechanism which, in response to tremor, deforms the prism, or steers the prism, to maintain a constant line of sight. Thus, as the binoculars move down due to the tremor, the prism optical axis is displaced upward such that the line of sight remains constant. This system, however, only compensates for the jitter of the binoculars and does not counter or eliminate the jitter induced by the person holding the binoculars. While the image does not appear to be moving, the binoculars continue to move. Only the line of sight is stabilized.
Open loop stabilization systems do exist for large scale bodies to eliminate disturbances such as movement over the ocean. This is done using a brute force approach to resist all movement. An enormous gyroscopic wheel is provided, for example under the floor of a sea going vessel, which spins continuously up to such a great speed that all rotation of the floor is resisted. This system however, would not be practical for a surgeon or a sniper. First, the gyroscopic wheel required would be too big compared to the rifle or the scalpel. Second, this type of system resists all movement, especially rapid movement which is necessarily what a surgeon or sniper may encounter, such as when the surgeon moves the scalpel across the patient, or the sniper moves to acquire a second target or follows a primary target.
Another open loop system stabilizes a gun, for example the gun on a tank, with respect to a platform, such as the tank body or the ground, as the tank moves across the terrain. The gun moves with respect to the tank, the jitter of the gun is sensed, and actuators push on the tank (platform) to offset movement of the barrel.
Yet another system intended to stabilize photography equipment uses gyroscopic wheels in opposing axes which spin continuously at 22,000 revolution's per minute to resist both pitch and yaw. However, this system requires an inverter or 400 cycle power access to run the device. Such power requirements prevent long range use in the field such as for a military or law enforcement sniper. Additionally, it takes 5 to 7 minutes before the required operating speed is attained. This system is thus hardly practical when the application is a sniper who must make quick and often optimistic shots. Moreover, because the system operates at a constant 22,000 revolution's per minute to resist any movement, it does not allow for rapid movement of the system which would be required by a sniper when the sniper must track a target or quickly acquire a second target.
The open loop systems discussed above stabilize a body with respect to a platform. However, there are systems which do not have such a platform to push against. For example, satellites in space use a closed loop, active attitude control system to maintain the satellite antenna in a constant orientation with respect to earth in order to maintain communications. This is accomplished using gyroscopic wheels which continuously spin faster and faster to provide a constant, 15 degree per hour rate of angular velocity as the satellite moves with respect to the earth. The motors, rather than pushing on a platform, push on a mass so that, due to conservation of momentum, the satellite will turn. However, that system only provides for the proper attitude of the satellite antenna with respect to a receiving station on earth and does not eliminate jitter of the satellite in response to any external disturbances.